<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="#" method="post" name="aaa">
  <input type="text" name="MM" value="2" id="MM">

</form>
<a href="#" onclick="go()" >Go</a>
</body>
<script>
  function go() {
    document.getElementById('MM').value="1";
  }
</script>
</html>